The disclosed technology relates to a fluid useful as a lubricant or a functional fluid, especially lubricating a vehicle, in particular an off-road vehicle such as a farm tractor, or a vehicle comprising a manual transmission. The technology involves the use of an amphiphilic diblock polymer as a water tolerance agent.
Traditional lubricants used for many off-road vehicle applications, and in particular farm tractor applications, are based on engine oil lubricants, despite the fact that farm tractor applications have additional technical lubricating requirements beyond those in lubricating a gasoline or diesel internal combustion engine. Reflecting their engine-oil heritage, most farm tractor lubricants include relatively large quantities of overbased metal detergents. The detergents typically are beneficial for providing a low level of static coefficient of friction, which is desirable for lubricating the metal-composition interface of the wet-brake component of a farm tractor. Detergents also aid in water tolerance, that is, adequate and stable dispersion of water contamination within the lubricant. However, high levels of detergents have been associated with excessive wear. Also characteristic of an engine oil, zinc dialkyldithiophosphates (ZDDPs) have typically been included in farm tractor fluid as antiwear agents to address wear problems. However, ZDDPs may cause corrosion problems, especially in the presence of water. Moreover, ZDDPs can cause friction durability issues with transmission clutch material and wet brake material under high-energy-producing conditions.
It is desired to provide a farm tractor fluid that is free from zinc dialkyldithiophosphates (or has a significantly reduced level thereof) in order to reduce or eliminate the corrosion problem and other problems. Then, however, to reduce or eliminate the wear problem, which would be expected to become more severe without the ZDDP, it is also desired to reduce the amount of detergent. Yet with these changes it remains necessary to retain a low static coefficient of friction (normally provided by the detergent) for lubricating the wet brake, all the while retaining antioxidancy properties and water tolerance.
A number of materials have been used in an attempt to provide water tolerance to lubricants. Among these are materials that have commonly been used as dispersants in engine lubricants, and in particular certain hydrocarbyl-substituted succinic acid esters. For instance, the condensation product of polyisobutylenesuccinic anhydride and diethylaminoethanol has been used to improve water tolerance in certain lubricant formulations, but it alone is often insufficient.
In short, it is a very difficult challenge to find a suitable balance, among competing performance requirements and component properties, in order to design a fully satisfactory farm tractor lubricant.
U.S. Patent Application 2006/0189490 discloses a lubricating composition containing base oil and at least one additive having friction-modifying properties. The additive is a linear diblock copolymer which includes hydrophobic segments P and polar segments D, said hydrophobic segments being obtained by polymerization of monomer compositions which comprise 0 to 40% of C1-5 alkyl (meth)acrylates, 50 to 100% of C6-30 alkyl (meth)acrylates, and 0 to 50% of a polar group containing ester, thioester or amide functionality.
International publication WO2006/047393 discloses linear and star RAFT polymers as viscosity index improvers in a variety of lubricants. The RAFT polymers may have a variety of architectures including diblock copolymers. All of polymers are derived from C12-15 alkyl (meth)acrylates.